The study utilizes three models to assess platelet aggregation associated with systemic trauma, shock and local trauma. Platelet aggregation will be assessed using autologous 51Cr tagged platelets and coagulation at various sites will be determined by measuring 125I tagged fibrinogen and these will be coupled with techniques to determine tissue plasminogen activator activity. A rat trauma model will be evaluated to determine the effects of splenectomy, low molecular weight Dextran and aspirin on platelet sequestration in the lung, liver, and muscle. The effects of hypophysectomy on the process, particularly of platelet sequestration in the lung, will be evaluated. Centrineurogenic etiology of the respiratory distress syndrome will be evaluated and the sequence of events determined. The role of platelet sequestration in changes in hemodynamics will be evaluated and the importance of the hypophyseal axis in mediating the effects of increased intracranial pressure will be assessed. Finally, the protective effect of aspirin and dipyrimidole will be evaluated in small vessels (less than 2 mm) and the length of time that the medications are required to prevent late occlusion will be assessed.